Flash memory cells using a floating gate to store charges thereon and memory arrays of such non-volatile memory cells formed in a semiconductor substrate are well known in the art. Typically, such floating gate memory cells have been of the split gate type, or stacked gate type.
One prior art non-volatile memory cell 10 is shown in FIG. 1. The split gate SuperFlash (SF) memory cell 10 comprises a semiconductor substrate 1 of a first conductivity type, such as P type. The substrate 1 has a surface on which there is formed a first region 2 (also known as the source line SL) of a second conductivity type, such as N type. A second region 3 (also known as the drain line) also of a second conductivity type, such as N type, is formed on the surface of the substrate 1. Between the first region 2 and the second region 3 is a channel region 4. A bit line (BL) 9 is connected to the second region 3. A word line (WL) 8 (also referred to as the select gate) is positioned above a first portion of the channel region 4 and is insulated therefrom. The word line 8 has little or no overlap with the second region 3. A floating gate (FG) 5 is over another portion of the channel region 4. The floating gate 5 is insulated therefrom, and is adjacent to the word line 8. The floating gate 5 is also adjacent to the first region 2. A coupling gate (CG) 7 (also known as control gate) is over the floating gate 5 and is insulated therefrom. An erase gate (EG) 6 is over the first region 2 and is adjacent to the floating gate 5 and the coupling gate 7 and is insulated therefrom. The erase gate 6 is also insulated from the first region 2.
One exemplary operation for erase and program of prior art non-volatile memory cell 10 is as follows. The cell 10 is erased, through a Fowler-Nordheim tunneling mechanism, by applying a high voltage on the erase gate EG 6 with other terminals equal to zero volt. Electrons tunnel from the floating gate FG 5 into the erase gate EG 6 causing the floating gate FG 5 to be positively charged, turning on the cell 10 in a read condition. The resulting cell erased state is known as ‘1’ state. Another embodiment for erase is by applying a positive voltage Vegp on the erase gate EG 6, a negative voltage Vcgn on the coupling gate CG 7, and applying a zero voltages on other terminals. The negative voltage Vcgn couples negatively the floating gate FG 5, hence less positive voltage Vcgp is required for erasing. Electrons tunnel from the floating gate FG 5 into the erase gate EG 6 causing the floating gate FG 5 to be positively charged, turning on the cell 10 in a read condition (cell state ‘1’). Alternatively, the wordline WL 8 (Vwle) and the source line SL 2 (Vsle) can be negative to further reduce the positive voltage on the erase gate FG 5 needed for erase. The magnitude of negative voltage Vwle and Vsle in this case is small enough not to breakdown the surrounding oxide and not to forward the p/n junction.
The cell 10 is programmed, through a source side hot electron programming mechanism, by applying a high voltage on the coupling gate CG 7, a high voltage on the source line SL 2, a medium voltage or a voltage that is the same as the source line voltage on the erase gate EG 6, and a programming current on the bit line BL 9. A portion of electrons flowing across the gap between the word line WL 8 and the floating gate FG 5 acquire enough energy to inject into the floating gate FG 5 causing the floating gate FG 5 to be negatively charged, turning off the cell 10 in read condition. The resulting cell programmed state is known as ‘0’ state.
The cell 10 can be inhibited in programming (if, for instance, another cell in its row is to be programmed but cell 10 is to not be programmed) by applying an inhibit voltage on the bit line BL 9. A split gate flash memory operation and various circuitry are described in U.S. Pat. No. 7,990,773, “Sub Volt Flash Memory System,” by Hieu Van Tran, et al, and U.S. Pat. No. 8,072,815, “Array of Non-Volatile Memory Cells Including Embedded Local and Global Reference Cells and Systems,” by Hieu Van Tran, et al, which are incorporated herein by reference.
With reference to FIG. 2, flash memory cells of the type shown in FIG. 1 are arranged in an array. A row of flash memory cells is selected using a word line, and a column of flash memory cells is selected using a bit line. A specific flash memory cell is selected using a combination of a word line and a bit line. Flash memory cells are further arranged into sectors. The flash memory cells within a particular sector share an erase gate control line, and all flash memory cells within a particular sector are erased at the same time using the erase gate control line. The flash memory cells within a particular sector also share a source line. Flash memory cells within a particular row also share a control gate line.
In the illustrative example of FIG. 2, two sectors of flash memory cells are shown. It is to be understood that a flash memory array can include any number of sectors and that each sector can include any number of rows and columns of flash memory cells. In this example, each flash memory cell shown (201, 202, 203, 211, 212, 213, 221, 222, 223, 231, 232, and 233) follow the design of FIG. 1.
Sector 150 comprises cell 201, cell 202, and cell 203 in a row accessed by word line 151 and control gate 251 and cell 211, cell 212, and cell 213 in a row accessed by word line 152 and control gate 252. Sector 160 comprises cell 221, cell 222, and cell 223 in a row accessed by word line 161 and control gate 261 and cell 231, cell 232, and cell 233 in a row accessed by word line 162 and control gate 162. The cells in sector 150 are erased by erase gate line 155, and the cells in sector 160 are erased by erase gate line 165. The cells in sector 150 are coupled to source line 156, and the cells in sector 160 are coupled to source line 166.
For each cell, its respective bit line (101, 102, or 103) is attached to bit line 9 in FIG. 1, its word line (151, 152, 161, or 162) is attached to word line 8 in FIG. 1, its erase gate line (155 or 165) is attached to erase gate 6 in FIG. 1, its control gate line (251, 252, 261, or 262) is attached to control gate 7 in FIG. 1, and its source line (156 or 166) is coupled to source line 2 in FIG. 1.
In the prior art system of FIG. 2, two or more control gate lines within each sector are coupled to a control gate line decoder. Thus, in FIG. 2, control gate line 251 and control gate line 252 are coupled to control gate line decoder 250, and control gate line 261 and control gate line 262 are coupled to control gate line decoder 260. Control gate line decoder 250 can be coupled to control gate voltage source 255, and control gate line decoder 260 can be coupled to control gate voltage source 265. If, for example, the system desires to activate control gate line 252, it will configure control gate line decoder 250 to couple control gate line 252 to control gate voltage source 255. This configuration can occur using a selection signal (not shown) sent to control gate line decoder 250.
One undesired consequence of this prior art design is that disturbances will emerge during the programming process of flash memory cells due to the use of control gate line decoders within sectors. For example, if cell 212 is to be programmed, word line 152 and bit line 102 will be activated, and source line 156 will contain a high voltage. Control line decoder 250 will couple control gate line 252 to control gate voltage source 255. In actual operation, some charge will leak from control gate voltage source 255 through control line decoder 250 to control gate line 251. This will have the unintended consequence of sometimes programming cell 202 (known as a column disturbance) due to its sharing of bit line 102 and source line 156 with cell 212, and of sometimes programming cell 203 (known as a diagonal disturbance) and possibly other cells in sector 150 due to their sharing of source line 156 with cell 212. In addition, cell 213 also will sometimes be unintentionally programmed (known as a row disturbance) due to its sharing of word line 152, control gate line 252, and source line 156 with cell 212.
What is needed is an improved system that minimizes the occurrences of disturbances during the programming of flash memory cells.